To design and select materials for silicone hydrogel lenses, many factors must be considered to optimize the physical, chemical and biological properties. Examples of these properties include oxygen permeability, wettability, lubricity, biocompatibility, physical strength, modulus, and optical requirements, to name just a few.
While patient comfort has driven the market use of silicone hydrogel lenses, the utility of these lenses depends on both the physical properties, such as oxygen permeability, water content, low Young's modulus, and lubricity of the lens, as well as the amount of protein and lipid deposition on the lenses during wear. A recent review by Tighe (A Decade of Silicone Hydrogel Development: Surface Properties, Mechanical Properties, and Ocular Compatibility, Eye and Contact Lens 39, 3-11 (2013)) noted that two important trends in contact lenses in the last decade were decreasing modulus and increasing water content. A low modulus contact lens is important because lower modulus lenses are more likely to conform to eye curvature, thereby resulting in improved user comfort. It was also noted that the trend in modulus reduction was not solely as a result of changes in water content.
Although silicone hydrogel lenses have been used extensively over the last 10 years, several difficulties with silicone hydrogel lenses are still encountered. These difficulties stem from the fact that silicone is a hydrophobic material.
For this reason, silicone lenses tend to develop a relatively hydrophobic, non-wettable surface in contact with a hydrophobic lens mold during manufacturing. Compatibilizing of hydrophilic and hydrophobic components within silicone hydrogel formulations is critical for the manufacturing of optically clear, wettable contact lenses. For instance, optical clarity may be negatively impacted by phase separation of hydrophobic silicone from hydrophilic components in the lens formulation or in the final lens saturated with aqueous media.
In addition, lipids and proteins have a high tendency to deposit on a hydrophobic surface. The deposits may negatively affect optical clarity and wear properties. Furthermore, adsorption of unwanted components from the ocular tear fluid on to the lens during wear is a contributory factor for reduced user comfort. In addition, bacterial infections can potentially occur if lens care regimens are not followed. The extent of undesirable adsorptions on the lens will determine the lens care needs for a specific lens and will impact the duration that the lens can be present in the eye.
Some of these difficulties can be alleviated by improving the hydrophilicity of the lens. Many approaches have been used to improve the hydrophilicity of the silicone based contact lenses. These include introducing wetting agents, using hydrophilic monomers, and synthesis of copolymers of silicone and hydrophilic polymers or oligomers.
In the synthesis of copolymers, one common approach has been to create block copolymers of silicones and a hydrophilic polymer. The hydrophilic polymer is typically polyethylene glycol (PEG) due to its availability with hydroxyl and amine terminated groups, which allows straightforward preparation of block copolymers.